1. Field of the Invention
The present invention relates to semiconductor lasers and methods for manufacturing the same.
2. Description of the Related Art
In the field of optical communication systems in recent years, the amount of transmitting information is drastically increasing. In order to respond to the increase in the amount of transmitting information, wavelength-division multiplexing (WDM) systems have been constructed. In WDM systems, optical signals with different wavelengths are transmitted through a single optical waveguide such as an optical fiber. In WDM systems, a plurality of semiconductor lasers with different lasing wavelengths are conventionally used as light sources. The lasing wavelengths of these semiconductor lasers are fixed in the conventional WDM systems. A more efficient communication system can be constructed by making the wavelengths of the semiconductor lasers tunable. High performances such as high wavelength stability, a wide wavelength tunable range, compactness, low power consumption, and high-speed operation are required for wavelength tunable semiconductor lasers.
For example, U.S. Pat. No. 4,896,325 discloses a wavelength tunable semiconductor laser with DBR regions including sampled gratings (SGs). A sampled grating has a periodic structure in which a segment of a grating is periodically separated by a blank space at a sampling period. FIG. 15 is a schematic cross-sectional view showing the configuration of a semiconductor laser 100 discussed in U.S. Pat. No. 4,896,325. The semiconductor laser 100 includes four regions provided on a single semiconductor substrate, namely, a gain region 151, a phase control region 152, and distributed Bragg reflector (DBR) regions 153 and 154. In this semiconductor laser 100, optical light with a relatively broad wavelength band is generated at an active layer 161 in the gain region 151 and a light with a specific wavelength is selectively emitted.
The DBR regions 153 and 154 respectively include sampled gratings (SGs) 163 and 164 and each have a reflection spectrum having a plurality of reflectivity peaks at a fixed wavelength interval. Moreover, the interval between the reflectivity peaks in the SG 163 is different from the interval between the reflectivity peaks in the SG 164. Laser oscillation is achieved at a wavelength where the reflectivity peak of the SG 163 overlaps with the reflectivity peak of the SG 164. For an SG, the wavelength of the reflectivity peaks is determined on the basis of a grating period, and the wavelength interval between the reflectivity peaks is determined on the basis of the sampling period of SG.
By injecting current into the DBR regions 153 and 154, the refractive indices of the SGs 163 and 164 can be changed. As a result, the wavelengths at the reflectivity peaks are shifted, thereby controlling the lasing wavelength by utilizing a so-called vernier effect. The phase control region 152 controls the phase of light guiding through the optical waveguide 162 in the phase control region 152 so as to perform fine adjustment on the lasing wavelength. In the semiconductor laser 100 having the above configuration, the refractive indices of the SGs 163 and 164 and a waveguide 162 are appropriately adjusted by current injection, thereby allowing for a continuous change in the lasing wavelength.